Characterization of crustal xenoliths from the Bearpaw Mountains, Montana (USA), using U-Pb geochronology, whole-rock geochemistry and thermobarometry, with implications for lower crustal processes and evolution of the Wyoming Craton

Thakurdin, Yashirvad; Bolhar, Robert; Horváth, Péter; Rocholl, Alexander; Collerson, K. D.

doi:10.1016/j.chemgeo.2019.05.034

Cited by 20 publications

(1 citation statement)

References 90 publications

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“…This indicates that, relative to the bulk rock, the thin sections have higher proportions of quartz and plagioclase feldspar, which contain little to no Fe, Mg and most incompatible trace elements, and lower proportions of the most abundant mafic phase, hornblende. Lower crustal xenoliths, by contrast to gneisses from terrains, are commonly granoblastic and have massive equilibrated textures and relatively simple mineralogies (Figure 6) [36][37][38]. This is primarily because xenoliths are entrained by rapidly rising magmas rather than being tectonically exhumed, improving survival of massive, granoblastic textures.…”

Section: Evaluation Of the Reconstitution Approachmentioning

confidence: 99%

A Reconstitution Approach for Whole Rock Major and Trace Element Compositions of Granulites from the Kapuskasing Structural Zone

Emo

Kamber

2020

Minerals

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Current estimates for the composition of the lower continental crust show significant variation for the concentrations of the highly incompatible elements, including large uncertainties for the heat-producing elements. This has consequences for models of the formation of lower crust. For example, is lower continental crust inherently poor in incompatible elements or has it become so after extraction of partial melts caused by thermal incubation? Answering these questions will require better agreement between estimates for the chemistry of the lower crust. One issue is that granulite samples may have been altered during ascent. Xenoliths often experience contamination from the entraining alkaline magma, potentially resulting in elevated concentrations of incompatible trace elements when analysed by conventional bulk rock techniques. To avoid this, we assessed an in situ approach for reconstructing whole rock compositions with granulites from the Kapuskasing Structural Zone, Superior Province, Canada. As terrain samples, they have not been affected by host magma contamination, and as subrecent glacial exposures, they show minimal modern weathering. We used scanning electron microscope electron dispersive spectroscopy (SEM-EDS) phase mapping to establish the modal mineralogy. Major and trace element concentrations of mineral phases were determined by electron microprobe and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), respectively. These concentrations were combined with the modal mineralogies to obtain reconstructed whole rock compositions, which were compared to conventional bulk rock analyses. The reconstructed data show good reproducibility relative to the conventional analyses for samples with massive textures. However, the conventional bulk rock chemistry systematically yields higher K concentrations, which are hosted in altered feldspars. Thus, even in terrain samples, minor alteration can lead to elevated incompatible element estimates that may not represent genuine lower continental crust.

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Section: Evaluation Of the Reconstitution Approachmentioning

confidence: 99%

A Reconstitution Approach for Whole Rock Major and Trace Element Compositions of Granulites from the Kapuskasing Structural Zone

Emo

Kamber

2020

Minerals

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A 3-billion-year history of magmatism, metamorphism, and metasomatism recorded by granulite-facies xenoliths from central Montana, USA

Ringwood,

Ortner,

Seward

et al. 2024

Contrib Mineral Petrol

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Lower crustal xenoliths from the Missouri Breaks diatremes and Bearpaw Mountains volcanic field in Montana record a multi-billion-year geologic history lasting from the Neoarchean to the Cenozoic. Unusual kyanite-scapolite-bearing mafic granulites equilibrated at approximately 1.8 GPa and 890 °C and 2.3 GPa and 1000 °C (67 and 85 km depth) and have compositions pointing to their origin as arc cumulates, while metapelitic granulites record peak conditions of 1.3 GPa and 775 °C (48 km depth). Rutile from both mafic granulites and metapelites have U-Pb dates that document the eruption of the host rocks at ca. 46 Ma (Big Slide in the Missouri Breaks) and ca. 51 Ma (Robinson Ranch in the Bearpaw Mountains). Detrital igneous zircon in metapelites date back to the Archean, and metamorphic zircon and monazite record a major event beginning at 1800 Ma. Both zircon and monazite from a metapelite from Robinson Ranch also document an earlier metamorphic event at 2200–2000 Ma, likely related to burial/metamorphism in a rift setting. Metapelites from Big Slide show a clear transition from detrital igneous zircon accumulation to metamorphic zircon and monazite growth around 1800 Ma, recording arc magmatism and subsequent continent-continent collision during the Great Falls orogeny, supporting suggestions that the Great Falls tectonic zone is a suture between the Wyoming craton and Medicine Hat block. U-Th-Pb and trace-element depth profiles of zircon and monazite record metasomatism of the lower crust during the Laramide orogeny at ~60 Ma, bolstering recent research pointing to Farallon slab fluid infiltration during the orogeny.

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Evolution of continental crust and sedimentary rock chemistry through time

Kamber,

Ossa Ossa

2025

Treatise on Geochemistry

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Characterization of crustal xenoliths from the Bearpaw Mountains, Montana (USA), using U-Pb geochronology, whole-rock geochemistry and thermobarometry, with implications for lower crustal processes and evolution of the Wyoming Craton

Cited by 20 publications

References 90 publications

A Reconstitution Approach for Whole Rock Major and Trace Element Compositions of Granulites from the Kapuskasing Structural Zone

A Reconstitution Approach for Whole Rock Major and Trace Element Compositions of Granulites from the Kapuskasing Structural Zone

A 3-billion-year history of magmatism, metamorphism, and metasomatism recorded by granulite-facies xenoliths from central Montana, USA

Evolution of continental crust and sedimentary rock chemistry through time

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